This guide is for upgrading the sling. Get to Chapter 10. While it's loading, press and hold
until the chapter is loaded, then wait for a few seconds. Now, hold
, then highlight an already-crafted upgrade (if you don't have one yet for that row, highlight the locked one to the right. Then, while holding
like you normally would to craft ammunion, quickly select the one you want to craft, then keep holding
until the entire row is upgraded.
Here's a video showing it working:
I take no credit in the making of this video.